Simplify df(1) by factoring out most of the common code:
Simplify df(1) by factoring out most of the common code:

 - In the argc == 0 case, just populate the mount list as before, but
   do not calculate widths, update totals or print anything.

 - In the argv > 0 case, collect information about the requested file
   systems and store it in the mount list, but do not calculate
   widths, update totals or print anything.

 - In either case, once all the information has been collected,
   iterate once through the mount list to calculate widths and totals,
   then once more to print everything.

This also fixes two bugs: firstly, column widths were not calculated
correctly if more than one file system was specified on the command
line; and secondly, file systems with MNT_IGNORE were included in the
totals even if -a was not specified.

Noticed by:	Paul Schenkeveld
MFC after:	3 weeks

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